Chemicals used to extend the life and usefulness of wood products are extremely important to wood users. Durability of wood in homes, service buildings, animal holding pens and fence posts is a major concern to almost every homeowner and farmer.

Wood-preserving chemicals have made possible a wide variety of otherwise unsuitable applications. Without preservatives, frequent maintenance and replacement would make wood too costly for electrical power poles, bridges, railroad ties, home decks, porches and other uses subjected to weathering and decay. Thus, preservatives help American companies compete in a variety of market applications.

Chromated Copper Arsenate

Chromated copper arsenate (CCA) is the most common preservative in use today. In the treating process CCA chemicals are applied under pressure and the chemicals are driven deep into the wood cells. Done properly, pressure-treated wood can last up to 40 years, compared to teh 3 to 7 year life span of similar untreated posts.

CCA works by making the wood poisonous to the fungi and insects that use it as a food source. The coppor component of CCA is the main toxin for fungi, and arsenic poisons the few species that are copper-tolerant. Insect suchs as termites are poisoned by the arsenic that is digested out of the treated wood they feed upon.

Chromium is the agent responsible for binding the arsenic and cooper to the molecules that makes up woods. This process renders the toxic principals water-insoluble, and they cannot leach out of the wood. EPA and U.S. Department of Agriculture studies have shown that CCA-treated lumber is safely used in the presence of plants and animals. Plants do not absorb the chemicals and animals that chew on teh wood cannot digest out the arsenic. However, these chemicals can be released as toxic gases if the wood is burned. There is also the possibility of skin and lung iritation from contact with surface residues or sawdust created during construction.

Although studies have indicated CCA-treated wood is safe for direct contact with food, the Food and Drug Administration has not approved its use for structures or containers such as beehives, feed bins or silos, where the feed or food would come into direct contact with the wood.

Handling and Using Treated-Wood Products

The United States Environmental Protection Agency (EPA) has reclassified certain wood-preserving chemicals--creosote, pentachloropophenol and inorganic arsenicals--as restricted use pesticide materials. The designation means they are toxic or poisonous and potentially hazardous to humans and animals. Consequently, the public should be aware of the precautions for using wood treated with these chemicals.

Use of Protective Equipment

Frequent or prolonged skin contact with pentachloropehnol or creosote treated wood should be avoided. When handling pesticide treated wood, wear tightly woven coveralls and use gloves impervious to these chemicals (vinyl gloves). Handlers of inorganic arsenical treated wood do not need to wear such gloves because of the negligible rate of arsenic absorption through human skin.

Sawdust from treated wood tends to become airborne with resultant human exposure by inhalation, swallowing and widespread dermal contact. To reduce the potential for exposure, the EPA recommends that individuals sawing pesticide treated wood in their occupation should wear disposable coveralls, made of a material such as nitrile or polyethylene, or similar protecctive clothing. Homeowners with occasional exposure should wear tightly woven long-sleeved cotton overalls, if they do not have disposable coveralls.

Individuals who are sawing pesticide treated wood should wear a dust mask capable of trapping 80% of particulates greater than 5 microns in size. In addition, sawing operations should be performed outdoors to prevent indoor accumulations of airborne sawdust and goggles should be worn to protect eyes from flying particles.

Skin should be washed after contact with pesticides, especially before eating or drinking. Protective clothing should be laundered before reuse if oily preservative or sawdust has accumulated on them. Work clothes should be washed separately from other household clothing.

Disposal of Treated Wood

Treated wood should not be burned in open fires, in stoves or in fireplaces because toxic cehmicals may be produced in the smoke and ashes. However, larger quantities of treated wood from commercial and industrial uses, such as construction sites, may be burned in commerical or industrial incinerators if done in accordance with state and federal regulations.

Small quantities of treated wood can be disposed of by burial or by ordinary trash collection and disposal methods. Burial of treated wood does not pose unreasonable risks to human health and the environment.

Indoor Use of Treated Wood

Pentachlorophenol treated wood should not be used in residential, industrial or commercial building interiors, except in laminated beams or building components that contact the ground and are subject to decay or insect infestation. When used for such purposes, pentachlorophenol treated wood must be covered with two coats of an appropriate sealer such as urethane, shellac, latex epoxy enamel or varnish.

The EPA has advised that creosote treated wood should not be used in residential building interiors. Creosote treated wood in interiors of industrial buildings should be used only for wood block flooring and for industrial building components that contact the soil and are subject ot decay or insect infestation. Such industrial application is acceptable, provided that two coats of an appropriate sealer are applied. For creosote treated wood block floaring, coal tar pitch and coal tar pitch emulsion are effective sealers. Urethane, epoxy and shellac are appropriate sealers for creosote treated wood when used other than as wood block flooring It is believed that limited interior uses of appropriately sealed pentachlorophenol and creosote treated wood will not cause unreasonable adverse effects to humans or the environment.

EPA has not advised against the interior use of arsenical treated wood because the arsenic air levels measured in homes constructed with this type of treated wood are not significantly different from background arsenic air levels when dust has been vacuumed from the wood surface. Therefore, wood that is pressure-treated with water-borned arsenical preservations may be used inside residences wtihout a sealer, providing that all machined sawdust is vacuumed from the wood surface.

Animal Contact

Several types of domestic animals can be exposed to the pesticides in treated wood by breathing air containing pesticide vapors or dust particles, by oral contact or by dermal contact. The principal problem is chemical residues being concentrated in the edible tissues of animals that humans may consume, rather than acute toxicity in the animals following direct exposure.

Regulations prohibit use of pentachlorophenol and creosote treated wood in interiors of farm buildings where the wood may come into contact with domestic animals or livestock may bite, rub, scrape or lick the wood. The intent is to prevent any exposure to the animals or to humans who may consume food products of animal origin.

In interiors of farm buildings where domestic animals or livestock are unlikely to bite or lick the wood, pentachlorophenol or creosote treated wood may be used for building components that contact the soil and that are subject to decay or insect infestation, and where two coats of an appropriate sealer are applied. For creosote treated wood, use urethane, epoxy or shellac; for pentachlorophenol treated wood, use urethane, shellac, latex epoxy enamel or varnish.

Studies have shown that arsenic is readily excreted from cattle and does not accumulate in the food chain, and that dermal absorption of arsenic from treated wood is negligible. If domestic animals do lick the wood, exposure is expected to be minimal. Therefore, EPA does not prohibit the use of arsenical treated wood in areas where it may have direct contact with domestic animals or livestock.

Food and Feed

The Food and Drug Administration (FDA) has the responsibility for setting food additive regulations. FDA has stated that preservative treated wood cannot be used under circumstances where the pesticide may become a component of food or feed, unless a food additive regulation prescribes safe use.

FDA also has expressed concern about the use of treated wood under conditions and in sites where chlorinated phenols from pentachlorophenol could migrate to animal feed and become concentrated in the edible tissue of animals. Examples of such sites should be structures or containers used for storing silage or other animal feeds.

No food additive regulation permits food or animal feed to contain pentachlorophenol, creosote or organic arsenicals at any concentration. Any food or animal feed containing any residues from contact with treated wood would be considered adulterated under the Food, Drug and Cosmetic Act.

The regulations also apply to food plants. Where treated wood timbers are used as retainers for raised vegetable gardens, vegetables may contact the treated wood surface. Covering the treated wood surface with polyethylene prevents potential food contamination from the treated wood.

EPA cautions against using treated wood in parts of beehvies that are likely to come into contact with honey and could result in pesticide residues in the honey.

Drinking Water for Humans

EPA has concluded that arsenic treated wood should not be used where it will come in contact with drinking water, until studies are conducted to show that the contribution of arsenic to drinking water via this source cannot be expected to exceed 5 parts per billion in the water. Areas where treated wood should not be used in drinking water systems include interiors of reservoir roofs, support beams and truss structures above water levels and may involve either direct or indirect contact with drinking water. In these cases, migration of preservative chemicals at significant rates is possible either by direct contact and leaching, when direct contact is involved and by condenstaion and other mechanisms when treated wood is in indirect contact with drinking water.

Because creosote and other coal tar based products impart an objectionable taste and odor to water, EPA recommends that creosote treated wood not be used where it will come in contact with drinkiing water.

Studies have indicated that water in contact with pentachlorophenol treated wood contained pentachlorophenol levels in excess of recognized safe levels. EPA has recommended that pentachlorophenol treated wood should not be used where it will come in contact wiht drinking water. An exception exists for incidental contact such as dock and bridge pilings.

Drinking Water for Domestic Animals

Pentachlorophenol and creosote treated wood should not be used where it may come in direct contact with drinking water for domestic animals and livestock. An exception involves incidental contact, such as dock and bridge pilings. However, arsenical treated wood may be used for construction of containers for water that will be consumed by domestic animals or livestock.

Outdoor Uses of Pressure Treated Wood

Research studies have found that dermal abosrption of inorganic arsenic from pressure treat wood is negligible. Consequently, this wood can be used as handrails, sundecks, park benches, stadium seats, boardwalks and playground equipment.

However, the skin absorption of pentachlorophenol and creosote is relatively high. EPA recommends that pentachlorophenol and creosote treated wood not be used where it might be in frequent or prolonged contact with the skin, unless two coats of an effective sealer have been applied to the wood (urethane, epoxy or shellac for creosote treated wood and urethane, shellac, latex epoxy enamel or varnish for pentachlorophenol treated wood). Chairs and other outdoor furniture are typical examples of such uses.

Adapted from PennState and Delaware Cooperative Extensions, 2001